1. Field of the Invention
The present invention relates to an alkaline storage battery.
2. Description of Related Art
An alkaline storage battery comprises an external casing enclosing a group of electrodes. The electrode group includes a nickel positive electrode and a negative electrode containing either cadmium hydroxide or hydrogen absorbing alloys as an active material. The positive electrode and the negative electrode are rolled or layered together with a separator between the two electrodes. The electrode group is impregnated with an alkaline electrolytic solution. A closure cap covers an opening of the external casing. A gasket usually made of nylon, is inserted between the closure cap and the external casing to seal the opening.
When compared with other storage batteries, an alkaline storage battery such as above has excellent charging/discharging characters and can be hermetically sealed so that this alkaline storage battery is used in a variety of appliances.
Improved performance, such as a larger capacity and a longer life, is required for such alkaline storage battery, and their research and development have been conducted to respond to this need.
As batteries are recently used in a wider variety of environments, the alkaline storage battery is also required to endure a wide temperature range from low to high temperatures.
To improve characteristics of the alkaline storage battery at a high temperature, Li is added to an electrolytic solution in one known technique (see Japanese Laid-Open Patent Application No. 11-219721), for instance.
Other techniques to improve the high-temperature characteristic of the alkaline battery are also known as follows:
(a) A solid solution is formed by Co and nickel hydroxide which is a positive electrode active material so as to suppress a decrease in charging efficiency at a high temperature.
(b) Cobalt hydroxide and yttrium hydroxide are added into nickel hydroxide, which is a major constituent of a positive electrode plate. A concentration of Y is 0.5xcx9c3%. This provides a high battery capacity and an excellent charging/discharging cycle character (see Japanese Laid-Open Patent Application No. 11-73957).
(c) A surface layer is provided to each particle formed by a crystal of a nickel hydroxide active material. This surface layer contains a solid solution formed by Co, Mn, and other elements like Mg, Ca, and Sr. As a result, high charging efficiency at a high temperature can be provided (see Japanese Laid-Open Patent Application. No. 10-125518).
(d) Al and V are included in a nickel active material. A layer containing Ca, Ti, and the like is formed on the surface of the nickel active material. This provides enhanced charging characteristic at a high temperature (see Japanese Laid-Open Patent Application No. 10-149821).
(e) Nickel hydroxide particles are covered by a layer made of Ni- and Y-base hydroxides so as to provide an enhanced characteristic at a high temperature to an active material (see Japanese Laid-Open Patent Application No. 10-255790).
The above techniques provide alkaline storage batteries with excellent characteristics at room temperatures and high temperatures. However, these alkaline storage batteries do not have a sufficient discharging capacity at a low temperature of around xe2x88x9240xc2x0 C. In this way, an alkaline storage battery that can be used with stability over a wide temperature range from low to high temperatures have not been achieved yet.
The present invention therefore aims to provide an alkaline storage battery that can be used with stability over a wide temperature range,
To achieve this object, the alkaline storage battery of the present invention includes; a nickel positive electrode containing one or tore types of additive compounds selected from a calcium compound, a strontium compound, a scandium compound, an yttrium compound, and a lanthanoid compound; and an alkaline electrolytic solution having an alkaline concentration of 10 moles/liter (mol/l) or higher.
When the nickel positive electrode includes the above additive compounds, an oxygen overvoltage at a high temperature-increases. This suppresses generation of oxygen gas, and therefore can promote a charging response more than other responses. As a result, charging characteristics at a high temperature enhance.
With a conventional alkaline storage battery, an alkaline concentration of an alkaline electrolytic solution is set as 9 mol/l or lower so as to maintain good operating voltage of the storage battery although a high alkaline concentration of 10 mol/l or higher is set for the present invention. Since an alkaline electrolytic solution with a high alkaline concentration has a low freezing point, the alkaline storage battery of the present invention can operate with stability at a low temperature of around xe2x88x9240xc2x0 C. and maintain a sufficient discharging capacity. It has been also proved that a sufficient operating voltage can be obtained when the alkaline concentration of the alkaline electrolytic solution is 10 mol/l or higher. In this way, the alkaline storage battery of the present invention has both a sufficient operating voltage and improved discharging characteristics at low temperatures.
A maximum allowable alkaline concentration of the alkaline electrolytic solution should be set within a-range in which an electrolyte compound in the electrolytic solution does not deposit at a low temperature although a very high alkaline concentration may lower an operating voltage of the battery. Accordingly, a suitable alkaline concentration should be set with consideration also given to the operating voltage. The experiments have proved that an alkaline voltage battery-having an alkaline concentration of 10 to 11 mol/l can operate with stability over a wide temperature range from a low temperature to a high temperature.
The alkaline electrolytic solution for the present invention preferably contains one or more types of compounds selected from cesium hydroxide, rubidium hydroxide, potassium hydroxide, and sodium hydroxide as an electrolyte. When lithium-hydroxide, for instance, that has a low water solubility is used for an alkaline electrolytic solution and its concentration is raised, the lithium hydroxide deposits. The above electrolyte compounds for the present invention, however, have a high water solubility, and therefore are not likely to deposit.
When a gasket made of polysulfone is inserted between an external casing and a closure cap of the above alkaline storage battery, a good battery performance can be maintained over a long period as polysulfone has excellent heat resistance.
In addition, as the alkaline storage battery of the present invention includes a separator made of two or more separator means that are layered together, an internal short circuit can be reduced. This is especially advantageous to a nickel-cadmium storage battery, whose negative electrode contains cadmium. When this storage battery is used at a high temperature, a cadmium dendrite grows and an internal short circuit is likely to occur. Accordingly, the above separator made of two or more separator members are suitable for maintaining good long-term performance of the battery.